1. Technical Field of the Invention
The invention relates to apparatus and accompanying methods for use in a Coriolis mass flow rate meter for providing a mass flow rate signal that has a substantially reduced harmonic content.
2. Description of the Prior Art
Currently, Coriolis mass flow rate meters are finding increasing use as an accurate way to measure the mass flow rate of various process fluids in many applications.
Generally speaking, a Coriolis mass flow rate meter, such as that described in U.S. Pat. No. 4,491,025 (issued to J. E. Smith et al on Jan. 1, 1985), contains one or two parallel conduits, each typically being a U-shaped flow conduit or tube. Each flow conduit is driven to oscillate about an axis to create a rotational frame of reference. For a U-shaped flow conduit, this axis can be termed the bending axis. As process fluid flows through each oscillating flow conduit, movement of the fluid produces reactionary Coriolis forces that are orthogonal to both the velocity of the fluid and the angular velocity of the conduit. These reactionary Coriolis forces cause each conduit to twist about a torsional axis that, for a U-shaped flow conduit, is normal to its bending axis. The amount of twist imparted to each conduit is related to the mass flow rate of the process fluid flowing therethrough. This twist is frequently measured using velocity signals obtained from magnetic velocity sensors that are mounted to one or both of the flow conduits in order to provide a complete velocity profile of the movement of each flow conduit with respect either to the other conduit or a fixed reference.
In such a meter, the mass flow rate of a fluid that moves through the meter is proportional to the time interval that elapses between the instant one point situated on a side leg of a flow conduit crosses a pre-determined location, e.g. a respective mid-plane of oscillation, until the instant a corresponding point situated on the opposite side leg of the same flow conduit, crosses its corresponding location, e.g. its respective mid-plane of oscillation. For parallel dual conduit Coriolis mass flow rate meters, this interval is equal to the phase difference between the velocity signals generated for both flow conduits at the fundamental (resonant) frequency at which the flow conduits are driven. Hence, a critical goal of Coriolis mass flow rate meter designs is to measure a time interval for conduit movement that occurs only at the fundamental frequency at which the flow conduits are being driven.
Traditionally, this time interval is measured by using traditional zero crossing or level detection techniques to detect the occurrence of corresponding points on both velocity sensor signals. I have observed that time interval measurements obtained in this fashion contain components resulting from harmonics of the fundamental driving frequency of the flow conduit. These harmonics are frequently caused by non-linearities existing in the mechanical metering assembly itself and/or in the magnetic velocity sensors. Unfortunately, these harmonics disadvantageously inject error into the time interval measurements which, in turn, contaminate the velocity signal produced by either tube sensor. This error causes the phase shift between the two flow conduit sensor signals to disadvantageously change from its true value and hence adversely affect the overall accuracy of the meter. Specifically, these time delays appear as a phase difference between velocity sensor signals for each of the two flow conduits. Harmonics, particularly those having a non-zero value at zero crossings of the fundamental flow conduit driving frequency, impart an error component that resembles a phase shift to each velocity waveform. This error component can increase significantly as the phase relationship of a given harmonic to the fundamental driving frequency changes. For example, certain harmonics, such as the third harmonic, may not be noticeable at certain phase differences but become quite prevalent at other phase differences. Inasmuch as the phase shift attributable to mass flow rate is often a small value, then any harmonic content may inject a noticeable error component into measured phase shift and thereby into the measurement of the actual mass flow rate of the fluid as it travels through the meter. Consequently, the measured time difference will not only contain a true phase shift component attributable to actual mass flow rate but also an error component due to harmonic content. Of all the harmonics of the driving frequency, the second harmonic imparts the largest error component. Mass flow rate measurements predicated on such time interval measurements will consequently contain an error component.
Although the art teaches several techniques aimed at removing harmonics in Coriolis mass flow rate meters, all of these techniques possess various drawbacks. In particular, in one technique that is often used, the zero (or level) crossing detectors are preceded by a bandpass filter, such as a well known Chebyshev or Butterworth type analog filter. Unfortunately, the output of these filters varies with temperature. Moreover, it is difficult to provide two such analog filters that are exactly matched to each other for temperature variations. Consequently, if two analog filters contain a slight temperature characteristic mis-match, as frequently occurs, any temperature variation disadvantageously will likely inject error into the phase and hence into the mass flow rate measurements. Another technique which is directed at eliminating this error inherent in analog filters involves converting the analog signals produced by the flow conduit sensors into a stream of digital values, digitally filtering these values and then reconverting the results back into the analog domain to measure the phase using conventional zero (or level) crossing detectors. Unfortunately, such an approach is quite complex and unnecessarily expensive to implement in a commercial meter.
Moreover, harmonics often do not occur under laboratory metering conditions. In fact, harmonics, particularly the second harmonic, do not occur under all flow conditions. Moreover, under field conditions, the actual harmonics that result in a given installation are often nearly impossible to predict. Furthermore, the density of the measured process fluid may and often does change from time to time. Consequently, the resonant frequency at which the flow conduits vibrate and the frequency of all the harmonics thereof will correspondingly shift with a density change. Consequently, the difficulty of predicting which harmonics, if any, will occur in any given field situation coupled with subsequent frequency shifts in these harmonics caused by density changes exacerbate and further complicate the problem of filtering these harmonics from the velocity sensor signals and/or time interval measurements.
Due to the apparent difficulty and attendant expense associated with the problem of adequately removing harmonics from velocity sensor signals and/or time interval measurements, it appears that the art has merely accepted the fact that such signals and measurements used in Coriolis mass flow rate meters will inherently contain harmonics that can not be readily removed or filtered. Since harmonics can generate errors that are simply unacceptably large in certain high accuracy metering applications, the art has chosen to accept a view that Coriolis mass flow rate meters inherently possess a limited accuracy which, in turn, precludes their use in certain applications for which they would otherwise be ideally suited.
Consequently, a need exists in the art for a highly accurate Coriolis mass flow rate meter and particularly one that produces a mass flow rate signal that has a substantially reduced harmonic content, i.e. frequencies other the fundamental frequency at which the flow conduit is driven.